Display apparatuses utilizing organic light emitting diodes (OLEDs), hereinafter referred to as OLED displays, have become a focus of attention as next-generation display apparatuses.
The OLED displays are classified into a bottom emission type and a top emission type, depending upon the direction in which light-emitting elements emit light, as disclosed in “Recent Trend of Flat Panel Display (2004)” by Toray Research Center.
The bottom emission type, formed with light-emitting elements on a transparent substrate, utilizes the light emitted toward the substrate by the light-emitting elements through the substrate, while the top emission type utilizes the light emitted in the opposite direction to the substrate.
The deterioration of the OLED is accelerated with driving time, so that a terminal-to-terminal resistance increases.
The deterioration appears more remarkable as the driving current becomes larger.
Accordingly, increasing a ratio (an aperture ratio) of a light-emitting area in each pixel of a display apparatus can extend the lifetime of the OLED while ensuring fixed light intensity as the display apparatus.
Moreover, making each pixel maintain its light intensity during frame periods can lower driving current while ensuring light intensity required for the display apparatus to function. Accordingly, active matrix drive technology also becomes essential for long life of the display apparatus.
In recent years, the active matrix drive technology of the OLED by a thin film transistor (TFT) has been researched and developed actively.
FIG. 1 illustrates the most basic pixel circuit in active matrix drive with a transistor having a channel layer of n-type semiconductor.
Each pixel has two transistors (a switching transistor and a driving transistor).
While a sufficiently high voltage Vsel (>0) is applied into a gate electrode of the switching transistor, conductivity between drain and source electrodes of the switching transistor is raised and a signal potential (=Vsig) is written into a gate electrode of the driving transistor.
Channel conductance of the driving transistor changes with the magnitude of Vsig to permit control of the intensity of light emission.
After Vsel is removed (Vsel application is stopped), conductivity between the drain and the source electrodes of the switching transistor is lowered, so that a signal potential written in the driving transistor is retained and the light-emitting element retains light emission with the fixed intensity corresponding to Vsig.
Moreover, in many cases, a storage capacitor is added in parallel to the gate electrode of the driving transistor.
This is because an influence of gate leak current of the driving transistor and an influence of parasitic capacitance of the driving transistor or the switching transistor is alleviated to stably retain a gate potential of the driving transistor over frame periods.
In other words, the storage capacitor is essential in ensuring gradation controllability of a pixel circuit.
In recent years, development of transistors using transparent conducting oxide polycrystalline thin films for channel layers is on the increase.
For example, Nature (Volume 432, 2004, Pages 488 to 492) has disclosed a transistor using a transparent amorphous oxide semiconductor film with a composition ratio by X-ray fluorescence analysis of In:Ga:Zn=1.1:1.1:0.9 for a channel layer.
Each of the source electrode, the drain electrode and the gate electrode is made of tin-doped indium oxide (ITO).
Furthermore, Proceedings of the 2nd International TFT Conference, 6.3, Pages 176 to 179 (2006) has disclosed a transistor with channel layer made of RF sputter thin film (In—Ga—Zn—O thin film) using polycrystalline InGaZnO4 target.
Each of the source electrode, the drain electrode and the gate electrode has a multilayered film made of titanium and gold. The above-described two types of transistors operate in an n-type enhancement mode.